Abstract-On-chip Terahertz-Frequency Measurements of Liquids

Matthew Swithenbank, Andrew David Burnett, Christopher Russell, Lianhe H. Li, Alexander Giles Davies, Edmund H. Linfield, John E. Cunningham, and Christopher David Wood

Anal. Chem., Just Accepted Manuscript

DOI: 10.1021/acs.analchem.7b01235

Publication Date (Web): July 6, 2017

Copyright © 2017 American Chemical Society

http://pubs.acs.org/doi/abs/10.1021/acs.analchem.7b01235?journalCode=ancham

Terahertz-frequency-range measurements can offer potential insight into the picosecond dynamics, and therefore function, of many chemical systems. There is a need to develop technologies capable of performing such measurements in aqueous and polar environments, particularly when it is necessary to maintain the full functionality of biological samples. In this study, we present a proof-of-concept technology comprising an on-chip planar Goubau line, integrated with a microfluidic channel, which is capable of low-loss, terahertz-frequency-range spectroscopic measurements of liquids. We also introduce a mathematical model that accounts for changes in the electric field distribution around the waveguide, allowing accurate, frequency-dependent liquid parameters to be extracted. We demonstrate the sensitivity of this technique by measuring a homologous alcohol series across the 0.1-0.8 THz frequency range.

Abstract-Free-space terahertz radiation from a LT-GaAs-on-quartz large-area photoconductive emitter

Free-space terahertz radiation from a LT-GaAs-on-quartz large-area photoconductive emitter

David R. Bacon, Andrew D. Burnett, Matthew Swithenbank, Christopher Russell, Lianhe Li, Christopher D. Wood, John Cunningham, Edmund H. Linfield, A. Giles Davies, Paul Dean, and Joshua R. Freeman

https://www.osapublishing.org/oe/abstract.cfm?uri=oe-24-23-26986

We report on large-area photoconductive terahertz (THz) emitters with a low-temperature-grown GaAs (LT-GaAs) active layer fabricated on quartz substrates using a lift-off transfer process. These devices are compared to the same LT-GaAs emitters when fabricated on the growth substrate. We find that the transferred devices show higher optical-to-THz conversion efficiencies and significantly larger breakdown fields, which we attribute to reduced parasitic current in the substrate. Through these improvements, we demonstrate a factor of ~8 increase in emitted THz field strength at the maximum operating voltage. In addition we find improved performance when these devices are used for photoconductive detection, which we explain through a combination of reduced parasitic substrate currents and reduced space-charge build-up in the device.

Published by The Optical Society under the terms of the Creative Commons Attribution 4.0 License. Further distribution of this work must maintain attribution to the author(s) and the published article's title, journal citation, and DOI.

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Abstract-The effect of molecular size and particle shape on the terahertz absorption of a homologous series of tetraalkylammonium salts.

Andrew David Burnett , John Kendrick , Christopher Russell , Jeppe Christensen , John E Cunningham ,Arwen R. Pearson , Edmund H Linfield , and A. Giles Davies

http://pubs.acs.org/doi/abs/10.1021/ac401657r

The absorption coefficient and refractive index have been measured for a homologous series of tetraalkylammonium bromides over the frequency range 0.3 to 5.5 THz. Spectral features are found to shift to lower frequencies as the molecular mass is increased, as expected. However, in order to understand the detailed structure of the observed spectral features, density functional perturbation theory calculations have been performed on the first four crystalline compounds in the series. From these calculations we find that each spectrum is dominated by three translatory modes involving asymmetric motion of the ammonium cation and bromine counter-ion, although the overall number of active modes increases with increasing molecular size. The experimentally observed absorption is not completely described by the infrared active phonon modes alone. We show that it is also necessary to include the coupling of the phonon modes with the macroscopic field generated by the collective displacement of the vibrating ions and we have applied an effective medium theory which accounts for particle shape to allow for this effect in the calculation of the terahertz spectra.